Environmental legislation on the use and emission of volatile organic components (VOC) is causing a change from conventional solvent-borne coatings to waterborne coatings. Even though the quality of currently available waterborne coatings is getting close to that of solvent-borne coatings, some significant drawbacks still exist. The most important quality difference is noticeable in waterborne non-pigmented coatings that are used in clear-coats, stains and varnishes on a variety of different substrates. Due to the fact that most waterborne coatings comprise waterborne binders that are composed of dispersed polymer particles, the wet coating will be opaque or milky so it lacks the in-can clarity (ICC) associated with solvent-borne coatings. This property is best described as the ability of visible light to pass through the polymer dispersion. The less light is absorbed and scattered by the vinyl polymer dispersion the better the ICC. The ICC is characterised by the transparency value measurement as described below.
Upon application of the coating, the wet coating layer will have a white to bluish appearance that will slowly disappear upon drying. In many cases the dry waterborne coating will also have a lower level of transparency compared to a solvent-borne coating. In a number of applications such as clear wood-coatings, these effects are highly undesirable so there is a need for waterborne coatings that have improved wet and dry coating clarity. One of the reasons for bad dry film clarity is insufficient film-formation or coalescence. In order to promote coalescence, waterborne coatings still contain significant amounts of VOC to reduce the minimal film-formation temperature (MFT) of the polymer dispersion.
When a waterborne coating is required that has good hardness, block and print resistance, the polymer dispersion should have a glass transition temperature (Tg) well above ambient temperature. In particular for such high Tg polymers, the level of VOC required for making sure that proper film formation takes place at ambient temperatures or below, will be too high to meet the current maximum level allowed by legislation. Therefore reduction of the MFT can no longer depend solely on the plasticizing effect of organic solvents.
A number of approaches have been developed to achieve reduction of MFT at ambient temperatures or below by designing the morphology of the dispersed polymer particles in vinyl polymers dispersions. Examples of such approaches are discussed by Heuts et al. in “Influence of morphology on the film formation of acrylic dispersions” ACS Symposium Series (1996), 648 (Film Formation in Waterborne Coatings), 271-285, where combinations of hard and soft polymer fractions either as blends or in sequential polymerization are described. However, having a significant amount of low Tg polymer present in the polymer composition will have a negative influence on the coating properties such as chemical resistance, hardness and block resistance.
EP0758364 (WO95-29963, Overbeek et al.) describes an aqueous composition having an optimized compromise of on one hand film forming properties and on the other hand coating properties, in particular Koenig hardness. The composition comprises a low molecular weight hydrophilic first stage polymer (oligomer). A draw-back of the process used is the fact that the oligomer has to be dissolved by the addition of a base prior to the polymerization of the second stage polymer. The alkaline solution of the oligomer acts as a polymeric dispersant stabilizing the second stage polymer. The oligomer is provided with crosslinkable groups to reduce water-sensitivity in the resulting coating caused by the low molecular weight hydrophilic oligomer. Further, inevitably the pH of the final polymer dispersion will be alkaline which is often not desirable for example when used in coatings for alkali sensitive substrates such as oak. Furthermore, the molecular weight of the oligomer must be relatively low to prevent that the viscosity of the alkaline solution will become too high or the dispersant action of the solubilized first stage polymer will be lost resulting in the formation of a product with a broad particle size distribution, lacking transparency and substantial formation of coagulum. Low molecular weight hydrophilic components are a disadvantage for the resulting coating properties.
EP 1008 635 B1 (Bardman et al.) describes a core-shell polymer where a first polymer contains from 0.5% to 7%. preferably from 1% to 5%, of a copolymerized monoethylenically unsaturated ionic monomer, based on the weight of the first polymer. A second polymer is polymerized in the presence of the first polymer. In this second polymer 0.25% to 6% (based on the weight of said second polymer) of copolymerized multi-ethylenically unsaturated monomer has to be present in the second stage. This patent uses a reactor pre-charge with a low concentration of inorganic salts, but there is no mention of the type of surfactant needed to obtain the desirable combination of properties. Even though a particle size from 30 to 500 nm is mentioned, all examples have particle sizes above 90 nm, so the dispersions do not have the required in can clarity. The use of chain-transfer agents is mentioned, but the patent is silent about the molecular weight range needed to have a good balance between minimal film-formation temperature and hardness.